ABSTRACT For more than 20 years, the National Longitudinal Study of Adolescent to Adult Health (Add Health), a nationally representative cohort of U.S. adolescents first contacted in 1994-95 who have now been followed into their fourth decade of life, has released all of its data to the scientific community. At present, Add Health has more than 30,000 users around the world across a range of scientific disciplines. Four waves of interviews have been conducted and a fifth wave is currently in the field. The comprehensive multilevel data includes survey responses, biological and genomic data, and social contextual data on friendship networks, romantic relationships, families, schools, neighborhoods, and the political, cultural, and built environment. Despite the widespread release and availability of Add Health data, updated documentation and metadata standards have not been applied to the rich longitudinal multilevel data. This application outlines a plan to substantially upgrade and expand the archiving, documentation, and dissemination of Add Health data. The intent is to make Add Health data even more easily accessible, straightforward to use, and clearly documented, while increasing awareness and facilitating expanded use of this national scientific resource. This project aims to: (1) enhance the archived Add Health data by applying the Data Documentation Initiative (DDI) international standard and harmonizing variables across the waves, thereby augmenting the information researchers can explore within the extensive collection of Add Health data; (2) generate searchable Add Health metadata in a human readable and machine-actionable format that will expand data discovery for the research community by providing researchers with robust web-based tools that facilitate the exploration of research questions that can be addressed with the Add Health data, generate customized codebooks that are unique to their research interests, and facilitate receipt of Add Health's public-use and restricted-use data; and (3) increase dissemination of Add Health data to the research community by depositing all waves of the Add Health public-use data in the NICHD Data and Specimen Hub, linking the metadata tool (described in Aim 2) to all repositories where Add Health's public-use data are deposited while integrating the new tools into the Carolina Population Center's Data Portal, and promoting researchers' awareness of the Add Health data and the data discovery tools. Add Health will be one of the first studies to apply DDI standards and provide metadata for multilevel social, behavioral, biological, genomic, and environmental data, contributing new innovations to the science behind DDI structure and methodology. Accomplishing the aims of this project will significantly expand the Add Health user base and multiply the scientific impact that Add Health has on social and biomedical research.